Abstract

Recent progress in radiometric dating and field mapping of volcanic rocks and in the development and refinement of plate-tectonic concepts has contributed greatly toward a better understanding of the spatial, temporal, and compositional distribution of Cenozoic volcanic deposits in the west-central United States.

Volcanism and subvolcanic Intrusive activity of early Cenozoic time may have been Influenced by the flattening of an eastward-dipping subducting slab of oceanic lithosphere. By the Eocene, andesitic volcanism erupted widely In the northern Rocky Mountains well Inland from the coastal trench region.

In middle Cenozoic time, andesitic volcanism waned in the Northern Rockies but intensified further south and was followed there by extrusion of voluminous silicic ash-flow sheets, caldera collapse, and shallow batholith emplacement. This activity may have been caused by a steepening or breakup of the subducting plate, by rising of hot, buoyant oceanic plate material to underplate the North American continent lithosphere, or by other means.

By late Cenozoic time, extension of the continental lithosphere Initiated block faulting, rifting, and associated basaltic or bimodal basaltic-rhyolitic volcanism. This activity continues today along the Snake River Plain-Yellowstone and Jemez zone systems as well as along the margins of the Great Basin.

Many volcanic to subvolcanic deposits of various Cenozoic ages are aligned along major northeast- and northwest-trending linear belts. Such zones include the Idaho-Montana porphyry belt, the Snake River Plain-Yellowstone system, the Colorado mineral belt, Jemez zone, Absaroka-Gallatin belt. Northern Black Hills zone and Trans-Pecos zone. These relationships suggest an interaction between subducting slabs and/or shearing stresses and deeply-penetrating discontinuities or flaws in the continental lithosphere.